Primary units, methods and systems for contact-less power transfer

ABSTRACT

There is disclosed a system and method for transferring power without requiring direct electrical conductive contacts. There is provided a primary unit having a power supply and a substantially laminar charging surface having at least one conductor that generates an electromagnetic field when a current flows therethrough and having an charging area defined within a perimeter of the surface, the at least one conductor being arranged such that electromagnetic field lines generated by the at least one conductor are substantially parallel to the plane of the surface or at least subtend an angle of 45° or less to the surface within the charging area; and at least one secondary device including at least one conductor that may be wound about a core. Because the electromagnetic field is spread over the charging area and is generally parallel or near-parallel thereto, coupling with flat secondary devices such as mobile telephones and the like is significantly improved in various orientations thereof.

This application is a divisional of application Ser. No. 10/514,046,filed on Feb. 28, 2005, which claims priority from UK patentapplications nos 0210886.8 of 13 May 2002, 0213024.3 of 7 Jun. 2002,0225006.6 of 28 Oct. 2002 and 0228425.5 of 6 Dec. 2002, as well as fromU.S. patent application Ser. No. 10/326,571 of 20 Dec. 2002. The fullcontents of all of these prior patent application is hereby incorporatedinto the present application by reference.

This invention relates to a new apparatus and method for transferringpower in a contact-less fashion.

Many of today's portable devices incorporate “secondary” power cellswhich can be recharged, saving the user the cost and inconvenience ofregularly having to purchase new cells. Example devices include cellulartelephones, laptop computers, the Palm 500 series of Personal DigitalAssistants, electric shavers and electric toothbrushes. In some of thesedevices, the cells are recharged via inductive coupling rather thandirect electrical connection. Examples include the Braun Oral B PlakControl power toothbrush, the Panasonic Digital Cordless Phone SolutionKX-PH15AL and the Panasonic multi-head men's shavers ES70/40 series.

Each of these devices typically has an adaptor or charger which takespower from mains electricity, a car cigarette lighter or other sourcesof power and converts it into a form suitable for charging the secondarycells. There are a number of problems associated with conventional meansof powering or charging these devices:

-   -   Both the characteristics of the cells within each device and the        means of connecting to them vary considerably from manufacturer        to manufacturer, and from device to device. Therefore users who        own several such devices must also own several different        adaptors. If users are going away on travel, they will have to        bring their collection of chargers if they expect to use their        devices during this time.    -   These adaptors and chargers often require users to plug a small        connector into the device or to place the device with accurate        alignment into a stand causing inconvenience. If users fail to        plug or place their device into a charger and it runs out of        power, the device becomes useless and important data stored        locally in the device might even be lost.    -   In addition, most adaptors and chargers have to be plugged into        mains sockets and hence if several are used together, they take        up space in plug strips and create a messy and confusing tangle        of wires.    -   Besides the above problems with conventional methods of        recharging devices, there are also practical problems associated        with devices having an open electrical contact. For example,        devices cannot be used in wet environments due to the        possibility of corroding or shorting out the contacts and also        they cannot be used in flammable gaseous environments due to the        possibility of creating electrical sparks.

Chargers which use inductive charging remove the need to have openelectrical contacts hence allowing the adaptor and device to be sealedand used in wet environments (for example the electric toothbrush asmentioned above is designed to be used in a bathroom). However suchchargers still suffer from all other problems as described above. Forexample, the devices still need to be placed accurately into a chargersuch that the device and the charger are in a predefined relativeposition (See FIGS. 1 a and 1 b). The adaptors are still only designedspecifically for a certain make and model of device and are still onlycapable of charging one device at a time. As a result, users still needto possess and manage a collection of different adaptors.

Universal chargers (such as the Maha MH-C777 Plus Universal charger)also exist such that battery packs of different shapes andcharacteristics can be removed from the device and charged using asingle device. Whilst these universal chargers eliminate the need forhaving different chargers for different devices, they create even moreinconvenience for the user in the sense that the battery packs firstneed to be removed, then the charger needs to be adjusted and thebattery pack needs to be accurately positioned in or relative to thecharger. In addition, time must be spent to determine the correct pairof battery pack metal contacts which the charger must use.

It is known from U.S. Pat. No. 3,938,018 “Induction charging system” toprovide a means for non-contact battery charging whereby an inductivecoil on the primary side aligns with a horizontal inductive coil on asecondary device when the device is placed into a cavity on the primaryside. The cavity ensures the relatively precise alignment which isnecessary with this design to ensure that good coupling is achievedbetween the primary and secondary coils.

It is also known from U.S. Pat. No. 5,959,433 “Universal InductiveBattery Charger System” to provide a non-contact battery chargingsystem. The battery charger described includes a single charging coilwhich creates magnetic flux lines which will induce an electricalcurrent in a battery pack which may belong to cellular phones or laptopcomputers.

It is also known from U.S. Pat. No. 4,873,677 “Charging Apparatus for anElectronic Device” to provide an apparatus for charging an electronicdevice which includes a pair of coils. This pair of coils is designed tooperate in anti-phase such that magnetic flux lines are coupled from onecoil to the other. An electronic device such as a watch can be placed onthese two coils to receive power.

It is also known from U.S. Pat. No. 5,952,814 “Induction chargingapparatus and an electronic device” to provide an induction charger forcharging a rechargeable battery. The shape of the external casing of theelectronic device matches the internal shape of the charger thusallowing for accurate alignment of the primary and secondary coils.

It is also known from U.S. Pat. No. 6,208,115 “Battery substitute pack”to provide a substitute battery pack which may be inductively recharged.

It is known from WO 00/61400 “Device for Inductively TransmittingElectrical Power” to provide a means of transferring power inductivelyto conveyors.

It is known from WO 95/11545 “Inductive power pick-up coils” to providea system for inductive powering of electric vehicles from a series ofin-road flat primaries.

To overcome the limitations of inductive power transfer systems whichrequire that secondary devices be axially aligned with the primary unit,one might propose that an obvious solution is to use a simple inductivepower transfer system whereby the primary unit is capable of emitting anelectromagnetic field over a large area (See FIG. 2 a). Users can simplyplace one or more devices to be recharged within range of the primaryunit, with no requirement to place them accurately. For example thisprimary unit may consist of a coil encircling a large area. When acurrent flows through the coil, an electromagnetic field extending overa large area is created and devices can be placed anywhere within thisarea. Although theoretically feasible, this method suffers from a numberof drawbacks. Firstly, the intensity of electromagnetic emissions isgoverned by regulatory limits. This means that this method can onlysupport power transfer at a limited rate. In addition, there are manyobjects that can be affected by the presence of an intense magneticfield. For example, data stored on credit cards maybe destroyed andobjects made of metal will have induced therein eddy currents generatingundesired heating effects. In addition, if a secondary device comprisinga conventional coil (see FIG. 2 a) is placed against a metallic platesuch as a copper plane in a printed circuit board or metallic can of acell, coupling is likely to be significantly reduced.

To avoid the generation of large magnetic fields, one might suggestusing an array of coils (See FIG. 3) whereby only the coils needed areactivated. This method is described in a paper published in the Journalof the Magnetics Society of Japan titled “Coil Shape in a Desk-typeContactless Power Station System” (29 Nov. 2001). In an embodiment ofthe multiple-coil concept, a sensing mechanism senses the relativelocation of the secondary device relative to the primary unit. A controlsystem then activates the appropriate coils to deliver power to thesecondary device in a localised fashion. Although this method provides asolution to the problems previously listed, it does so in a complicatedand costly way. The degree to which the primary field can be localisedis limited by the number of coils and hence the number of drivingcircuits used (i.e. the “resolution” of the primary unit). The costassociated with a multiple-coil system would severely limit thecommercial applications of this concept. Non-uniform field distributionis also a drawback. When all the coils are activated in the primaryunit, they sum to an equivalent of a large coil, the magnetic fielddistribution of which is seen to exhibit a minimum at the centre of thecoil.

Another scheme is outlined in U.S. Pat. No. 5,519,262 “Near Field PowerCoupling System”, whereby a primary unit has a number of narrowinductive coils (or alternatively capacitive plates) arranged from oneend to the other of a flat plate, creating a number of vertical fieldswhich are driven in a phase-shifted manner so that a sinusoidal wave ofactivity moves across the plate. A receiving device has two verticalfield pickups arranged so that regardless of its position on the plateit can always collect power from at least one pickup. While this schemealso offers freedom of movement of the device, it has the disadvantagesof needing a complex secondary device, having a fixed resolution, andhaving poor coupling because the return flux path is through air.

None of the prior art solutions can satisfactorily address all of theproblems that have been described. It would be convenient to have asolution which is capable of transferring power to portable devices withall of the following features and is cost effective to implement:

-   -   Universality: a single primary unit which can supply power to        different secondary devices with different power requirements        thereby eliminating the need for a collection of different        adaptors and chargers;    -   Convenience: a single primary unit which allows secondary        devices to be placed anywhere within an active vicinity thereby        eliminating the need for plugging-in or placing secondary        devices accurately relative to an adaptor or charger;    -   Multiple-load: a single primary unit that can supply power to a        number of secondary different devices with different power        requirements at the same time;    -   Flexibility for use in different environments: a single primary        unit that can supply power to secondary devices such that no        direct electrical contact is required thereby allowing for        secondary devices and the primary unit itself to be used in wet,        gaseous, clean and other atypical environments;    -   Low electromagnetic emissions: a primary unit that can deliver        power in a manner that will minimize the intensity and size of        the magnetic field generated.

It is further to be appreciated that portable appliances areproliferating and they all need batteries to power them. Primary cells,or batteries of them, must be disposed of once used, which is expensiveand environmentally unfriendly. Secondary cells or batteries can berecharged and used again and again.

Many portable devices have receptacles for cells of an industry-standardsize and voltage, such as AA, AAA, C, D and PP3. This leaves the userfree to choose whether to use primary or secondary cells, and of varioustypes. Once depleted, secondary cells must typically be removed from thedevice and placed into a separate recharging unit. Alternatively, someportable devices do have recharging circuitry built-in, allowing cellsto be recharged in-situ once the device is plugged-in to an externalsource of power.

It is inconvenient for the user to have to either remove cells from thedevice for recharging, or to have to plug the device into an externalpower source for recharging in-situ. It would be far preferable to beable to recharge the cells without doing either, by some non-contactmeans.

Some portable devices are capable of receiving power coupled inductivelyfrom a recharger, for example the Braun Oral B Plak Control toothbrush.Such portable devices typically have a custom, dedicated power-receivingmodule built-in to the device, which then interfaces with an internalstandard cell or battery (which may or may not be removable).

However it would be convenient if the user could transform any portabledevice which accepts industry-standard cell sizes into aninductively-rechargeable device, simply by fittinginductively-rechargeable cells or batteries, which could then berecharged in-situ by placing the device onto an inductive recharger.

Examples of prior art include U.S. Pat. No. 6,208,115, which discloses asubstitute battery pack which may be inductively recharged.

According to a first aspect of the present invention, there is provideda system for transferring power without requiring direct electricalconductive contacts, the system comprising:

-   i) a primary unit including a substantially laminar charging surface    and at least one means for generating an electromagnetic field, the    means being distributed in two dimensions across a predetermined    area in or parallel to the charging surface so as to define at least    one charging area of the charging surface that is substantially    coextensive with the predetermined area, the charging area having a    width and a length on the charging surface, wherein the means is    configured such that, when a predetermined current is supplied    thereto and the primary unit is effectively in electromagnetic    isolation, an electromagnetic field generated by the means has    electromagnetic field lines that, when averaged over any quarter    length part of the charging area measured parallel to a direction of    the field lines, subtend an angle of 45° or less to the charging    surface in proximity thereto and are distributed in two dimensions    thereover, and wherein the means has a height measured substantially    perpendicular to the charging area that is less than either of the    width or the length of the charging area; and-   ii) at least one secondary device including at least one electrical    conductor; wherein, when the at least one secondary device is placed    on or in proximity to a charging area of the primary unit, the    electromagnetic field lines couple with the at least one conductor    of the at least one secondary device and induce a current to flow    therein.

According to a second aspect of the present invention, there is provideda primary unit for transferring power without requiring directelectrical conductive contacts, the primary unit including asubstantially laminar charging surface and at least one means forgenerating an electromagnetic field, the means being distributed in twodimensions across a predetermined area in or parallel to the chargingsurface so as to define at least one charging area of the chargingsurface that is substantially coextensive with the predetermined area,the charging area having a width and a length on the charging surface,wherein the means is configured such that, when a predetermined currentis supplied thereto and the primary unit is effectively inelectromagnetic isolation, an electromagnetic field generated by themeans has electromagnetic field lines that, when averaged over anyquarter length part of the charging area measured parallel to adirection of the field lines, subtend an angle of 45° or less to thecharging surface in proximity thereto and are distributed in twodimensions thereover, and wherein the means has a height measuredsubstantially perpendicular to the charging area that is less thaneither of the width or the length of the charging area.

According to a third aspect of the present invention, there is provideda method of transferring power in a non-conductive manner from a primaryunit to a secondary device, the primary unit including a substantiallylaminar charging surface and at least one means for generating anelectromagnetic field, the means being distributed in two dimensionsacross a predetermined area in or parallel to the charging surface so asto define at least one charging area of the charging surface that issubstantially coextensive with the predetermined area, the charging areahaving a width and a length on the charging surface, the means having aheight measured substantially perpendicular to the charging area that isless than either of the width or the length of the charging area, andthe secondary device having at least one electrical conductor; wherein:

-   i) an electromagnetic field, generated by the means when energised    with a predetermined current and measured when the primary unit is    effectively in electromagnetic isolation, has electromagnetic field    lines that, when averaged over any quarter length part of the    charging area measured parallel to a direction of the field lines,    subtend an angle of 45° or less to the charging surface in proximity    thereto and are distributed in two dimensions over the at least one    charging area when averaged thereover; and-   ii) the electromagnetic field links with the conductor of the    secondary device when this is placed on or in proximity to the    charging area.

According to a fourth aspect of the present invention, there is provideda secondary device for use with the system, unit or method of the first,second or third aspects, the secondary device including at least oneelectrical conductor and having a substantially laminar form factor.

In the context of the present application, the word “laminar” defines ageometry in the form of a thin sheet or lamina. The thin sheet or laminamay be substantially flat, or may be curved.

The primary unit may include an integral power supply for the at leastone means for generating an electromagnetic field, or may be providedwith connectors or the like enabling the at least one means to beconnected to an external power supply.

In some embodiments, the means for generating the electromagnetic fieldhave a height that is no more than half the width or half the length ofthe charging area; in some embodiments, the height may be no more than ⅕of the width or ⅕ of the length of the charging area.

The at least one electrical conductor in the secondary device may bewound about a core that serves to concentrate flux therein. Inparticular, the core (where provided) may offer a path of leastresistance to flux lines of the electromagnetic field generated by theprimary unit. The core may be amorphous magnetically permeable material.In some embodiments, there is no need for an amorphous core.

Where an amorphous core is provided, it is preferred that the amorphousmagnetic material is a non-annealed or substantially as-cast state. Thematerial may be at least 70% non-annealed, or preferably at least 90%non-annealed. This is because annealing tends to make amorphous magneticmaterials brittle, which is disadvantageous when contained in a device,such as a mobile phone, which may be subjected to rough treatment, forexample by being accidentally dropped. In a particularly preferredembodiment, the amorphous magnetic material is provided in the form of aflexible ribbon, which may comprise one or more layers of one or more ofthe same or different amorphous magnetic materials. Suitable materialsinclude alloys which may contain iron, boron and silicon or othersuitable materials. The alloy is melted and then cooled so rapidly(“quenched”) that there is no time for it to crystallise as itsolidifies, thus leaving the alloy in a glass-like amorphous state.Suitable materials include Metglas® 2714A and like materials. Permalloyor mumetal or the like may also be used.

The core in the secondary device, where provided, is preferably a highmagnetic permeability core. The relative permeability of this core ispreferably at least 100, even more preferably at least 500, and mostpreferably at least 1000, with magnitudes of at least 10,000 or 100,000being particularly advantageous.

The at least one means for generating an electromagnetic field may be acoil, for example in the form of a length of wire or a printed strip, ormay be in the form of a conductive plate of appropriate configuration,or may comprise any appropriate arrangement of conductors. A preferredmaterial is copper, although other conductive materials, generallymetals, may be used as appropriate. It is to be understood that the term“coil” is here intended to encompass any appropriate electricalconductor forming an electrical circuit through which current may flowand thus generate an electromagnetic field. In particular, the “coil”need not be wound about a core or former or the like, but may be asimple or complex loop or equivalent structure.

Preferably, the charging area of the primary unit is large enough toaccommodate the conductor and/or core of the secondary device in aplurality of orientations thereof. In a particularly preferredembodiment, the charging area is large enough to accommodate theconductor and/or core of the secondary device in any orientationthereof. In this way, power transfer from the primary unit to thesecondary device may be achieved without having to align the conductorand/or core of the secondary device in any particular direction whenplacing the secondary device on the charging surface of the primaryunit.

The substantially laminar charging surface of the primary unit may besubstantially planar, or may be curved or otherwise configured to fitinto a predetermined space, such as a glove compartment of a cardashboard or the like. It is particularly preferred that the means forgenerating an electromagnetic field does not project or protrude aboveor beyond the charging surface.

A key feature of the means for generating an electromagnetic field inthe primary unit is that electromagnetic field lines generated by themeans, measured when the primary unit is effectively in magneticisolation (i.e. when no secondary device is present on or in proximityto the charging surface), are distributed in two dimensions over the atleast one charging area and subtend an angle of 45° or less to thecharging area in proximity thereto (for example, less than the height orwidth of the charging area) and over any quarter length part of thecharging area measured in a direction generally parallel to that of thefield lines. The measurement of the field lines in this connection is tobe understood as a measurement of the field lines when averaged over thequarter length of the charging area, rather than an instantaneous pointmeasurement. In some embodiments, the field lines subtend an angle of30° or less, and in some embodiments are substantially parallel to atleast a central part of the charging area in question. This is in starkcontrast to prior art systems, where the field lines tend to besubstantially perpendicular to a surface of a primary unit. Bygenerating electromagnetic fields that are more or less parallel to orat least have a significant resolved component parallel to the chargingarea, it is possible to control the field so as to cause angularvariations thereof, in or parallel to the plane of the charging area,that help to avoid any stationary nulls in the electromagnetic fieldthat would otherwise reduce charging efficiency in particularorientations of the secondary device on the charging surface. Thedirection of the field lines may be rotated through a complete orpartial circle, in one or both directions. Alternatively, the directionmay be caused to “wobble” or fluctuate, or may be switched between twoor more directions. In more complex configurations, the direction of thefield lines may vary as a Lissajous pattern or the like.

In some embodiments, the field lines may be substantially parallel toeach other over any given charging area, or at least have resolvedcomponents in or parallel to the plane of the charging area that aresubstantially parallel to each other at any given moment in time.

It is to be appreciated that one means for generating an electromagneticfield may serve to provide a field for more than one charging area; alsothat more than one means may serve to provide a field for just onecharging area. In other words, there need not be a one-to-onecorrespondence of means for generating electromagnetic fields andcharging areas.

The secondary device may adopt a substantially flat form factor with acore thickness of 2 mm or less. Using a material such as one or moreamorphous metal sheets, it is possible to have core thickness down to 1mm or less for applications where size and weight is important. See FIG.7 a.

In a preferred embodiment, the primary unit may include a pair ofconductors having adjacent coplanar windings which have mutuallysubstantially parallel linear sections arranged so as to produce asubstantially uniform electromagnetic field extending generally parallelto or subtending an angle of 45° or less to the plane of the windingsbut substantially at right angles to the parallel sections.

The windings in this embodiment may be formed in a generally spiralshape, comprising a series of turns having substantially parallelstraight sections.

Advantageously, the primary unit may include first and second pairs ofconductors which are superimposed in substantially parallel planes withthe substantially parallel linear sections of the first pair arrangedgenerally at right angles to the substantially parallel linear sectionsof the second pair, and further comprising a driving circuit which isarranged to drive them in such a way as to generate a resultant fieldwhich rotates in a plane substantially parallel to the planes of thewindings.

According to a fifth aspect of the present invention, there is provideda system for transferring power in a contact-less manner consisting of:

-   -   a primary unit consisting of at least one electrical coil        whereby each coil features at least one active area whereby two        or more conductors are substantially distributed over this area        in such a fashion that it is possible for a secondary device to        be placed in proximity to a part of this active area where the        net instantaneous current flow in a particular direction is        substantially non-zero;    -   at least one secondary device consisting of conductors wound        around a high permeability core in such a fashion that it is        possible for it to be placed in proximity to an area of the        surface of the primary unit where the net instantaneous current        flow is substantially non-zero;        whereby the at least one secondary device is capable of        receiving power by means of electromagnetic induction when the        central axis of the winding is in proximity to the active area        of the primary unit, is substantially not perpendicular to the        plane of the active area of primary unit and is substantially        not parallel to the conductors in the active area of at least        one of the coils of the primary unit.

Where the secondary device comprises an inductively rechargeable batteryor cell, the battery or cell may have a primary axis and be capable ofbeing recharged by an alternating field flowing in the primary axis ofthe battery or cell, the battery or cell consisting of:

-   -   an enclosure and external electrical connections similar in        dimensions to industry-standard batteries or cells    -   an energy-storage means    -   an optional flux-concentrating means    -   a power-receiving means    -   a means of converting the received power to a form suitable for        delivery to outside the cell through the external electrical        connections, or to recharge the energy storage means, or both.

The proposed invention is a significant departure from the design ofconventional inductive power transfer systems. The difference betweenconventional systems and the proposed system is best illustrated bylooking at their respective magnetic flux line patterns. (See FIGS. 2 aand 4)

-   -   Conventional System: In a conventional system (See FIG. 2 a),        there is typically a planar primary coil which generates a        magnetic field with flux lines coming out of the plane in a        perpendicular fashion. The secondary device has typically a        round or square coil that encircles some or all of these flux        lines.    -   Proposed system: In the proposed system, the magnetic field        travels substantially horizontally across the surface of the        plane (see FIG. 4) instead of directly out of the plane as        illustrated in FIG. 2 a. The secondary device hence may have an        elongated winding wound around a magnetic core. See FIGS. 7 a        and 7 b. When the secondary device is placed on the primary        unit, the flux lines would be attracted to travel through the        magnetic core of the secondary device because it is the lowest        reluctance path. This causes the secondary device and the        primary unit to be coupled effectively. The secondary core and        winding may be substantially flattened to form a very thin        component.

In describing the invention, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose.

It is to be understood that the term “charging area” used in this patentapplication may refer to the area of the at least one means forgenerating a field (e.g. one or more conductors in the form of a coil)or an area formed by a combination of primary conductors where thesecondary device can couple flux effectively. Some embodiments of thisare shown in FIGS. 6 a to 6 l and 9 c as component 740. A feature of a“charging area” is a distribution of conductors over a significant areaof the primary unit configured such that it is possible for the at leastone means for generating a field to be driven to achieve aninstantaneous net flow of flux in one direction. A primary unit may havemore than one charging area. One charging area is distinct from anothercharging area when flux cannot be effectively coupled by the secondarydevice (such as those shown in FIG. 7 a) in any rotation at theboundary.

It is to be understood that the term “coil” used in this patent refersto all conductor configurations which feature a charging area asdescribed above. This includes windings of wire or printed tracks or aplane as shown in FIG. 8 e. The conductors may be made of copper, gold,alloys or any other appropriate material.

The present application refers to the rotation of a secondary device inseveral places. It is to be clarified here that if a secondary device isrotated, the axis of rotation being referred to is the one perpendicularto the plane of the charging area.

This radical change in design overcomes a number of drawbacks ofconventional systems. The benefits of the proposed invention include:

-   -   No need for accurate alignment: The secondary device can be        placed anywhere on a charging area of the primary unit;    -   Uniform coupling: In the proposed invention, the coupling        between the primary unit and secondary device is much more        uniform over the charging area compared to a conventional        primary and secondary coil. In a conventional large coil system        (see FIG. 2 a), the field strength dips to a minimum at the        centre of the coil, in the plane of the coil (see FIG. 2 b).        This implies that if sufficient power is to be effectively        transferred at the centre, the field strength at the minimum has        to be above a certain threshold. The field strength at the        maximum will then be excessively higher than the required        threshold and this may cause undesirable effects.    -   Universality: a number of different secondary devices, even        those having different power requirements, can be placed within        charging areas on the charging surface of the primary unit to        receive power simultaneously;    -   Increased coupling coefficiency: Optional high permeability        magnetic material present in the secondary device increases the        induced flux significantly by offering a low reluctance path.        This can significantly increase the power transfer.    -   Desirable form factor for secondary device: The geometry of the        system allows thin sheets of magnetic material (such as        amorphous metal ribbons) to be used. This means that secondary        devices can have the form factor of a thin sheet, making it        suitable to be incorporated at the back of mobile phones and        other electronic devices. If magnetic material was to be used in        the centre of conventional coils, it is likely to increase the        bulkiness of the secondary device.    -   Minimised field leakage: When one or more secondary devices are        present in the charging area of the primary unit, it is possible        to use magnetic material in such a way that more than half of        the magnetic circuit is low reluctance magnetic material (see        FIG. 4 d). This means that more flux flows for a given        magneto-motive force (mmf). As the induced voltage is        proportional to the rate of change of flux linked, this will        increase the power transfer to the secondary device. The fewer        and shorter the air gaps are in the magnetic circuit, the less        the field will fringe, the closer the flux is kept to the        surface of the primary unit and hence leakage is minimized.    -   Cost effectiveness: Unlike the multiple-coil design, this        solution requires a much simpler control system and fewer        components.    -   Free axial rotation of secondary device: If the secondary device        is thin or optionally even cylindrical (see FIG. 10), it may be        constructed such that it continues to couple well to the flux        regardless of its rotation about its longest axis. This may in        particular be an advantage if the secondary device is a battery        cell fitted within another device, when its axial rotation may        be difficult to control.    -   The magnetic core in the secondary device may be located near        other parallel planes of metal within or near the device, for        example a copper printed circuit board or aluminium cover. In        this case, the performance of embodiments of the present        invention is significantly better than that of a conventional        core-wound coil because the field lines through a conventional        device coil will suffer flux-exclusion if the coil is placed up        against the metal plane (because the lines of flux must travel        perpendicular to the plane of the coil). Since in embodiments of        the present invention the lines of flux travel along the plane        of the core, and therefore also of the metal plane, performance        is improved. An additional benefit is that the magnetic core in        a secondary device of embodiments of the present invention can        act as a shield between the electromagnetic field generated by        the primary unit and any items (e.g. electrical circuits,        battery cells) on the other side of the magnetic core.    -   Because its permeability is higher than that of air, the        magnetic core of the secondary device of embodiments of the        present invention acts to concentrate magnetic flux, thus        capturing more flux than would otherwise flow through an        equivalent cross-section of air. The size of the core's “shape        factor” (the equivalent flux-capturing sphere) is determined to        a first-order approximation by the longest planar dimension of        the core. Therefore if the core of the secondary device of        embodiments of the present invention has planar dimensions with        a significantly non-square aspect ratio, for example a 4:1        rectangle instead of a 1:1 square, it will capture        proportionally more of any flux travelling parallel to the        direction of its longest planar dimension. Therefore if used in        devices which have a constrained aspect ratio (for example a        long thin device such as a headset or pen), a significant        increase in performance will be experienced compared with that        of a conventional coil of the same area.

The primary unit typically consists of the following components. (SeeFIG. 5)

-   -   Power supply: This power supply converts mains voltage into a        lower voltage dc supply. This is typically a conventional        transformer or a switch-mode power supply;    -   Control unit: The control unit serves the function of        maintaining the resonance of the circuit given that the        inductance of the means for generating a field changes with the        presence of secondary devices. To enable this function, the        control unit may be coupled to a sensing unit which feeds back        the current status of the circuit. It may also be coupled to a        library of capacitors which may be switched in and out as        required. If the means for generating a field requires more than        one driving circuit, the control unit may also coordinate the        parameters such as the phase difference or on/off times of        different driving circuits such that the desired effect is        achieved. It is also possible for the Q (quality factor) of the        system to be designed to function over a range of inductances        such that a need for the above control system is eliminated;    -   Driving circuit: The driving unit is controlled by the control        unit and drives a changing current through the means for        generating a field or a component of the means. More than one        driving circuit may be present depending on the number of        independent components in the means;    -   Means for generating an electromagnetic field: The means uses        current supplied from the driving circuits to generate        electromagnetic fields of pre-defined shapes and intensities.        The exact configuration of the means defines the shape and        intensity of the field generated. The means may include magnetic        material to act as flux guides and also one or more        independently driven components (windings), together forming the        charging area. A number of embodiment designs are possible and        examples are shown in FIG. 6.    -   Sensing unit: The sensing unit retrieves and sends relevant data        to the control unit for interpretation.

The secondary device typically consists of the following components, asshown in FIG. 5.

-   -   Magnetic unit: the magnetic unit converts the energy stored in        the magnetic field generated by the primary unit back into        electrical energy. This is typically implemented by means of a        winding wound around a highly permeable magnetic core. The        largest dimension of the core typically coincides with the        central axis of the winding.    -   Conversion unit: the conversion unit converts the fluctuating        current received from the magnetic unit into a form that is        useful to the device that it is coupled to. For example, the        conversion unit may convert the fluctuating current into an        unregulated dc supply by means of a full-wave bridge rectifier        and smoothing capacitor. In other cases, the conversion unit may        be coupled to a heating element or a battery charger. There is        also typically a capacitor present either in parallel or in        series with the magnetic unit to form a resonant circuit at the        operating frequency of the primary unit.

In typical operation, one or more secondary devices are placed on top ofthe charging surface of the primary unit. The flux flows through the atleast one conductor and/or core of the secondary devices present andcurrent is induced. Depending on the configuration of the means forgenerating a field in the primary unit, the rotational orientation ofthe secondary device may affect the amount of flux coupled.

The Primary Unit

The primary unit may exist in many different forms, for example:

-   -   As a flat platform or pad which can sit on top of tables and        other flat surfaces;    -   Built in to furniture such as desks, tables, counters, chairs,        bookcases etc. such that the primary unit may not be visible;    -   As part of an enclosure such as a drawer, a box, a glove        compartment of a car, a container for power tools;    -   As a flat platform or pad which can be attached to a wall and        used vertically.

The primary unit may be powered from different sources, for example:

-   -   A mains AC power outlet    -   A vehicle lighter socket    -   Batteries    -   Fuel Cells    -   Solar Panel    -   Human power

The primary unit may be small enough such that only one secondary devicemay be accommodated on the charging surface in a single charging area,or may be large enough to accommodate many secondary devicessimultaneously, sometimes in different charging areas.

The means for generating a field in the primary unit may be driven atmains frequency (50 Hz or 60 Hz) or at some higher frequency.

The sensing unit of the primary unit may sense the presence of secondarydevices, the number of secondary devices present and even the presenceof other magnetic material which is not part of a secondary device. Thisinformation may be used to control the current being delivered to thefield generating means of the primary unit.

The primary unit and/or the secondary device may be substantiallywaterproof or explosion proof.

The primary unit and/or the secondary device may be hermetically sealedto standards such as IP66.

The primary unit may incorporate visual indicators (for example, but notlimited to, light emitting devices, such as light emitting diodes,electrophosphorescent displays, light emitting polymers, or lightreflecting devices, such as liquid crystal displays or MITs electronicpaper) to indicate the current state of the primary unit, the presenceof secondary devices or the number of secondary devices present or anycombination of the above.

The Means for Generating an Electromagnetic Field

The field generating means as referred to in this application includesall configurations of conductors where:

-   -   The conductors are substantially distributed in the plane and;    -   Substantial areas of the plane exist where there is a non-zero        net instantaneous current flow. These are areas on which, given        the correct orientation, the secondary devices will couple        effectively and receive power. (See FIG. 6)    -   The conductors are capable of generating an electromagnetic        field where the field lines subtend an angle of 45° or less or        are substantially parallel to a substantial area of the plane.

FIG. 6 illustrate some possibilities for such a primary conductor.Although most of the configurations are in fact coil windings, it is tobe appreciated that the same effect can also be achieved with conductorplanes which are not typically considered to be coils (See FIG. 6 e).These drawings are typical examples and are non-exhaustive. Theseconductors or coils may be used in combination such that the secondarydevice can couple effectively in all rotations whilst on the chargingarea(s) of the primary unit.

Magnetic Material

It is possible to use magnetic materials in the primary unit to enhanceperformance.

-   -   Magnetic material may be placed below one or more charging areas        or the entire charging surface such that there is also a low        reluctance path on the underside of the conductors for the flux        to complete its path. According to theory, an analogy can be        drawn between magnetic circuits and electrical circuits. Voltage        is analogous to magneto-motive force (mmf), resistance is        analogous to reluctance and current is analogous to flux. From        this, it can be seen that for a given mmf, flux flow will        increase if the reluctance of the path is decreased. By        providing magnetic material to the underside of the charging        area, the reluctance of the magnetic circuit is essentially        decreased. This substantially increases the flux linked by the        secondary device and ultimately increases the power transferred.        FIG. 4 d illustrates a sheet of magnetic material placed        underneath the charging area and the resulting magnetic circuit.    -   Magnetic material may also be placed above the charging surface        and/or charging area(s) and below the secondary devices to act        as a flux guide. This flux guide performs two functions:        Firstly, the reluctance of the whole magnetic circuit is further        decreased allowing more flux to flow. Secondly, it provides a        low reluctance path along the top surface of the charging        area(s) so the flux lines will flow through these flux guides in        favour of flowing through the air. Hence this has the effect of        containing the field close to the charging surface of the        primary unit instead of in the air. The magnetic material used        for flux guides may be strategically or deliberately chosen to        have different magnetic properties to the magnetic core (where        provided) of the secondary device. For example, a material with        lower permeability and higher saturation may be chosen. High        saturation means that the material can carry more flux and the        lower permeability means that when a secondary device is in        proximity, a significant amount of flux would then choose to        travel through the secondary device in favour of the flux guide.        (See FIG. 8)    -   In some configurations of the primary unit field generating        means, there may be conductors present that do not form part of        the charging area, such as the component marked 745 in FIGS. 6 a        and 6 b. In such cases, one may wish to use magnetic material to        shield the effects of these conductors.    -   Examples of some materials which may be used include but are not        limited to: amorphous metal (metallic glass alloys such as        MetGlas™), mesh of wires made of magnetic material, steel,        ferrite cores, mumetal and permalloy.        The Secondary Device

The secondary device may take a variety of shapes and forms. Generally,in order for good flux linkage, a central axis of the conductor (forexample, a coil winding) should be substantially non-perpendicular tothe charging area(s).

-   -   The secondary device may be in the shape of a flattened winding.        (See FIG. 7 a) The magnetic core inside can consist of sheets of        magnetic material such as amorphous metals. This geometry allows        the secondary device to be incorporated at the back of        electronic devices such as mobile phones, personal digital        assistants and laptops without adding bulk to the device.    -   The secondary device may be in the shape of a long cylinder. A        long cylindrical core could be wound with conductors (See FIG. 7        b).    -   The secondary device may be an object with magnetic material        wrapped around it. An example is a standard-sized (AA, AAA,        C, D) or other sized/shaped (e.g. dedicated/customised for        particular applications) rechargeable battery cell with for        example magnetic material wrapped around the cylinder and        windings around the cylindrical body.    -   The secondary device may be a combination of two or more of the        above. The above embodiments may even be combined with a        conventional coil.

The following non-exhaustive list illustrates some examples of objectsthat can be coupled to a secondary device to receive power.Possibilities are not limited to those described below:

-   -   A mobile communication device, for example a radio, mobile        telephone or walkie-talkie;    -   A portable computing device, for example a personal digital        assistant or palmtop or laptop computer;    -   Portable entertainment devices, for example a music player,        games console or toy;    -   Personal care items, for example a toothbrush, shaver, hair        curler, hair rollers;    -   A portable imaging device, for example a video camcorder or a        camera;    -   Containers of contents that may require heating, for example        coffee mugs, plates, cooking pots, nail-polish and cosmetic        containers;    -   Consumer devices, for example torches, clocks and fans;    -   Power tools, for example cordless drills and screwdrivers;    -   Wireless peripheral devices, for example wireless computer        mouse, keyboard and headset;    -   Time keeping devices, for example clock, wrist watch, stop watch        and alarm clock;    -   A battery-pack for insertion into any of the above;    -   A standard-sized battery cell.

In the case of unintelligent secondary devices such as a battery cell,some sophisticated charge-control means may also be necessary to meterinductive power to the cell and to deal with situations where multiplecells in a device have different charge states. Furthermore, it becomesmore important for the primary unit to be able to indicate a “charged”condition, since the secondary cell or battery may not be easily visiblewhen located inside another electrical device.

A possible system comprising an inductively rechargeable battery or celland a primary unit is shown in FIG. 10. In addition to the freedom toplace the battery 920 freely in (X, Y) and optionally rotate it in rZ,relative to the primary unit 910, the battery can also be rotated alongits axis rA while continuing to receive power.

When a user inserts a battery into a portable device, it is not easy toensure that it has any given axial rotation. Therefore, embodiments ofthe present invention are highly advantageous because they can ensurethat the battery can receive power while in any random orientation aboutrA.

The battery or cell may include a flux concentrating means that may bearranged in a variety of ways:

-   -   1. As shown in FIG. 11 a, a cell 930 may be wrapped in a        cylinder of flux-concentrating material 931, around which is        wrapped a coil of wire 932.        -   a. The cylinder may be long or short relative to the length            of the cell.    -   2. As shown in FIG. 11 b, a cell 930 may have a portion of        flux-concentrating material 931 on its surface, around which is        wrapped a coil of wire 932.        -   a. The portion may be conformed to the surface of the cell,            or embedded within it.        -   b. Its area may be large or small relative to the            circumference of the cell, and long or short relative to the            length of the cell.    -   3. As shown in FIG. 11 c, a cell 930 may contain a portion of        flux-concentrating material 931 within it, around which is        wrapped a coil of wire 932.        -   a. The portion may be substantially flat, cylindrical,            rod-like, or any other shape.        -   b. Its width may be large or small relative to the diameter            of the cell        -   c. Its length may be large or small relative to the length            of the cell

In any of these cases, the flux-concentrator may be a functional part ofthe battery enclosure (for example, an outer zinc electrode) or thebattery itself (for example, an inner electrode).

Issues relating to charging of secondary cells (e.g. AA rechargeablecells in-situ within an appliance include:

-   -   Terminal voltage could be higher than normal.    -   Cells in series may behave strangely, particularly in situations        where some cells are charged, others not.    -   Having to provide enough power to run the device and charge the        cell.    -   If fast-charging is effected incorrectly, the cells may be        damaged.

Accordingly, some sophisticated charge-control means to meter inductivepower to the appliance and the cell is advantageously provided.Furthermore, it becomes more important for the primary unit to be ableto indicate a “charged” condition, since the secondary cell or batterymay not be easily visible when located inside an electrical device.

A cell or battery enabled in this fashion may be charged whilst fittedin another device, by placing the device onto the primary unit, orwhilst outside the device by placing the cell or battery directly ontothe primary unit.

Batteries enabled in this fashion may be arranged in packs of cells asin typical devices (e.g. end-to-end or side-by-side), allowing a singlepack to replace a set of cells.

Alternatively, the secondary device may consist of a flat “adapter”which fits over the batteries in a device, with thin electrodes whichforce down between the battery electrodes and the device contacts.

Rotating Electromagnetic Field

In the coils such as those in FIGS. 6, 9 a and 9 b, the secondarydevices will generally only couple effectively when the windings areplaced substantially parallel to the direction of net current flow inthe primary conductor as shown by the arrow 1. In some applications, onemight require a primary unit which will transfer power effectively tosecondary devices regardless of their rotation as long as:

-   -   the central axis of the secondary conductor is not perpendicular        to the plane and;    -   the secondary device is in close proximity to the primary unit

To enable this, it is possible to have two coils, for example onepositioned on top of the other or one woven into or otherwise associatedwith the other, the second coil capable of generating a net current flowsubstantially perpendicular to the direction of the first coil at anypoint in the active area of the primary unit. These two coils may bedriven alternately such that each is activated for a certain period oftime. Another possibility is to drive the two coils in quadrature suchthat a rotating magnetic dipole is generated in the plane. This isillustrated in FIG. 9. This is also possible with other combinations ofcoil configurations.

Resonant Circuits

It is known in the art to drive coils using parallel or series resonantcircuits. In series resonant circuits for example, the impedance of thecoil and the capacitor are equal and opposite at resonance, hence thetotal impedance of the circuit is minimised and a maximum current flowsthrough the primary coil. The secondary device is typically also tunedto the operating frequency to maximise the induced voltage or current.

In some systems like the electric toothbrush, it is common to have acircuit which is detuned when the secondary device is not present andtuned when the secondary device is in place. The magnetic materialpresent in the secondary device shifts the self-inductance of theprimary unit and brings the circuit into resonance. In other systemslike passive radio tags, there is no magnetic material in the secondarydevice and hence does not affect the resonant frequency of the system.These tags are also typically small and used far from the primary unitsuch that even if magnetic material is present, the inductance of theprimary is not significantly changed.

In the proposed system, this is not the case:

-   -   High permeability magnetic material may be present in the        secondary device and is used in close proximity to the primary        unit;    -   One or more secondary devices may be brought in close proximity        to the primary unit simultaneously.

This has the effect of shifting the inductance of the primarysignificantly and also to different levels depending on the number ofsecondary devices present on the pad. When the inductance of the primaryunit is shifted, the capacitance required for the circuit to resonant ata particular frequency also changes. There are three methods for keepingthe circuit at resonance:

-   -   By means of a control system to dynamically change the operating        frequency;    -   By means of a control system to dynamically change the        capacitance such that resonance is achieved at the predefined        frequency;    -   By means of a low Q system where the system remains in resonance        over a range of inductances.

The problem with changing the operating frequency is that the secondarydevices are typically configured to resonate at a predefined frequency.If the operating frequency changes, the secondary device would bedetuned. To overcome this problem, it is possible to change thecapacitance instead of the operating frequency. The secondary devicescan be designed such that each additional device placed in proximity tothe primary unit will shift the inductance to a quantised level suchthat an appropriate capacitor can be switched in to make the circuitresonate at a predetermined frequency. Because of this shift in resonantfrequency, the number of devices on the charging surface can be detectedand the primary unit can also sense when something is brought near ortaken away from the charging surface. If a magnetically permeable objectother than a valid secondary device is placed in the vicinity of thecharging surface, it is unlikely to shift the system to the predefinedquantised level. In such circumstances, the system could automaticallydetune and reduce the current flowing into the coil.

For a better understanding of the present invention and to show how itmay be carried into effect, reference shall now be made, by way ofexample only, to the accompanying drawings, in which:

FIG. 1 shows the magnetic design of typical prior art contact-less powertransfer systems which require accurate alignment of the primary unitand secondary device;

FIG. 2 a shows the magnetic design of another typical prior artcontact-less power transfer system which involves a large coil in theprimary unit;

FIG. 2 b shows the non-uniform field distribution inside the large coilat 5 mm distance from the plane of the coil, exhibiting a minimum in thecentre;

FIG. 3 shows a multiple-coil system where each coil is independentlydriven such that a localised field can be generated.

FIG. 4 a shows an embodiment of the proposed system which demonstrates asubstantial departure from prior art with no secondary devices present;

FIG. 4 b shows an embodiment of the proposed system with two secondarydevices present;

FIG. 4 c shows a cross section of the active area of the primary unitand the contour lines of the magnetic flux density generated by theconductors.

FIG. 4 d shows the magnetic circuit for this particular embodiment ofthe proposed invention;

FIG. 5 shows a schematic drawing of an embodiment of the primary unitand the secondary device;

FIGS. 6 a to 6 l show some alternative embodiment designs for the fieldgenerating means or a component of the field generating means of theprimary unit;

FIGS. 7 a and 7 b show some possible designs for the magnetic unit ofthe secondary device;

FIGS. 8 a–8 f show the effect of flux guides (the thickness of the fluxguide has been exaggerated for clarity);

FIG. 8 a shows that without flux guides, the field tends to fringe intothe air directly above the active area;

FIG. 8 b shows the direction of current flow in the conductors in thisparticular embodiment;

FIG. 8 c shows that the flux is contained within the flux guides whenmagnetic material is placed on top of the charging area;

FIG. 8 d shows a secondary device on top of the primary unit;

FIG. 8 e shows a cross section of the primary unit without any secondarydevices;

FIG. 8 f shows a cross section of the primary unit with a secondarydevice on top and demonstrates the effect of using a secondary core withhigher permeability than the flux guide.

FIG. 9 a shows a particular coil arrangement with a net instantaneouscurrent flow shown by the direction of the arrow;

FIG. 9 b shows a similar coil arrangement to FIG. 9 a except rotated by90 degrees;

FIG. 9 c shows the charging area of the primary unit if the coil of FIG.9 a is placed on top of FIG. 9 b. If the coil in FIG. 9 a if driven inquadrature to FIG. 9 b, the effect is a rotating magnetic dipole shownhere;

FIG. 10 shows the case where the secondary device has an axial degree ofrotation;

FIG. 11 shows various arrangements of secondary devices with axialdegrees of rotation;

FIG. 12 a and FIG. 12 b show another embodiment of the type of coilarrangement shown in FIG. 9 a and FIG. 9 b; and

FIG. 13 shows a simple embodiment of driving unit electronics.

Referring firstly to FIG. 1, there is shown two examples of prior artcontact-less power transfer systems which both require accuratealignment of a primary unit and a secondary device. This embodiment istypically used in electric toothbrush or mobile phone chargers.

FIG. 1 a shows a primary magnetic unit 100 and a secondary magnetic unit200. On the primary side, a coil 110 is wound around a magnetic core 120such as ferrite. Similarly, the secondary side consists of a coil 210wound around another magnetic core 220. In operation, an alternatingcurrent flows in to the primary coil 110 and generates lines of flux 1.When a secondary magnetic unit 200 is placed such that it is axiallyaligned with the primary magnetic unit 100, the flux 1 will couple fromthe primary into the secondary, inducing a voltage across the secondarycoil 210.

FIG. 1 b shows a split transformer. The primary magnetic unit 300consists of a U-shaped core 320 with a coil 310 wound around it. Whenalternating current flows into the primary coil 310, changing lines offlux are generated 1. The secondary magnetic unit 400 consists of asecond U-shaped core 420 with another coil 410 wound around it. When thesecondary magnetic unit 400 is placed on the primary magnetic unit 300such that the arms of the two U-shaped cores are in alignment, the fluxwill couple effectively into the core of the secondary 420 and inducevoltage across the secondary coil 410.

FIG. 2 a is another embodiment of prior art inductive systems typicallyused in powering radio frequency passive tags. The primary typicallyconsists of a coil 510 covering a large area. Multiple secondary devices520 will have voltage induced therein when they are within the areaencircled by the primary coil 510. This system does not require thesecondary coil 520 to be accurately aligned with the primary coil 510.FIG. 2 b shows a graph of the magnitude of magnetic flux intensityacross the area encircled by the primary coil 510 at 5 mm above theplane of the primary coil. It shows a non-uniform field, which exhibitsa minimum 530 at the centre of the primary coil 510.

FIG. 3 is another embodiment of a prior art inductive system wherein amultiple coil array is used. The primary magnetic unit 600 consists ofan array of coils including coils 611, 612, 613. The secondary magneticunit 700 may consist of a coil 710. When the secondary magnetic unit 700is in proximity to some coils in the primary magnetic unit 600, thecoils 611, 612 are activated while other coils such as 613 remaininactive. The activated coils 611, 612 generate flux, some of which willcouple into the secondary magnetic unit 700.

FIG. 4 shows an embodiment of the proposed invention. FIG. 4 a shows aprimary coil 710 wound or printed in such a fashion that there is a netinstantaneous current flow within the active area 740. For example, if adc current flows through the primary coil 710, the conductors in theactive area 740 would all have current flowing in the same direction.Current flowing through the primary coil 710 generates flux 1. A layerof magnetic material 730 is present beneath the charging area to providea return path for the flux. FIG. 4 b shows the same primary magneticunit as shown in FIG. 4 a with two secondary devices 800 present. Whenthe secondary devices 800 are placed in the correct orientation on topof the charging area 740 of the primary magnetic unit, the flux 1 willflow through the magnetic core of the secondary devices 800 instead offlowing through the air. The flux 1 flowing through the secondary corewould hence induce current in the secondary coil.

FIG. 4 c shows some contour lines for the flux density of the magneticfield generated by the conductors 711 in the charging area 740 of theprimary magnetic unit. There is a layer of magnetic material 730 beneaththe conductors to provide a low reluctance return path for the flux.

FIG. 4 d shows a cross-section of the charging area 740 of the primarymagnetic unit. A possible path for the magnetic circuit is shown. Themagnetic material 730 provides a low reluctance path for the circuit andalso the magnetic core 820 of the secondary magnetic device 800 alsoprovides a low reluctance path. This minimizes the distance the flux hasto travel through the air and hence minimizes leakage.

FIG. 5 shows a schematic drawing of an embodiment of the whole system ofthe proposed invention. In this embodiment, the primary unit consists ofa power supply 760, a control unit 770, a sensing unit 780 and anelectromagnetic unit 700. The power supply 760 converts the mains (orother sources of power) into a dc supply at an appropriate voltage forthe system. The control unit 770 controls the driving unit 790 whichdrives the magnetic unit 700. In this embodiment, the magnetic unitconsists of two independently driven components, coil 1 and coil 2,arranged such that the conductors in the charging area of coil 1 wouldbe perpendicular to the conductors in the charging area of coil 2. Whenthe primary unit is activated, the control unit causes a 90-degree phaseshift between the alternating current that flows through coil 1 and coil2. This creates a rotating magnetic dipole on the surface of the primarymagnetic unit 700 such that a secondary device is able to receive powerregardless of its rotational orientation (See FIG. 9). In standby modewhere no secondary devices are present, the primary unit is detuned andcurrent flow into the magnetic unit 700 is minimised. When a secondarydevice is placed on top of the charging area of the primary unit, theinductance of the primary magnetic unit 700 is changed. This brings theprimary circuit into resonance and the current flow is maximised. Whenthere are two secondary devices present on the primary unit, theinductance is changed to yet another level and the primary circuit isagain detuned. At this point, the control unit 770 uses feedback fromthe sensing unit 780 to switch another capacitor into the circuit suchthat it is tuned again and current flow is maximised. In thisembodiment, the secondary devices are of a standard size and a maximumof six standard-sized devices can receive power from the primary unitsimultaneously. Due to the standard sizes of the secondary devices, thechange in inductance due to the change in secondary devices in proximityis quantized to a number of predefined levels such that only a maximumof 6 capacitances is required to keep the system operating at resonance.

FIGS. 6 a to 6 l show a number of different embodiments for the coilcomponent of the primary magnetic unit. These embodiments may beimplemented as the only coil component of the primary magnetic unit, inwhich case the rotation of the secondary device is important to thepower transfer. These embodiments may also be implemented incombination, not excluding embodiments which are not illustrated here.For example, two coils illustrated in FIG. 6 a may be placed at 90degrees to each other to form a single magnetic unit. In FIGS. 6 a to 6e, the charging area 740 consists of a series of conductors with netcurrent generally flowing in the same direction. In certainconfigurations, such as FIG. 6 c, there is no substantial linkage whenthe secondary device is placed directly over the centre of the coil andhence power is not transferred. In FIG. 6 d, there is no substantiallinkage when the secondary device is positioned in the gap between thetwo charging areas 740.

FIG. 6 f shows a specific coil configuration for the primary unitadapted to generate electromagnetic field lines substantially parallelto a surface of the primary unit within the charging area 740. Twoprimary windings 710, one on either side of the charging area 740, areformed about opposing arms of a generally rectangular flux guide 750made out of a magnetic material, the primary windings 710 generatingopposing electromagnetic fields. The flux guide 750 contains theelectromagnetic fields and creates a magnetic dipole across the chargingarea 740 in the direction of the arrows indicated on the Figure. When asecondary device is placed in the charging area 740 in a predeterminedorientation, a low reluctance path is created and flux flows through thesecondary device, causing effective coupling and power transfer. It isto be appreciated that the flux guide 750 need not be continuous, andmay in fact be formed as two opposed and non-linked horseshoecomponents.

FIG. 6 g shows another possible coil configuration for the primary unit,the coil configuration being adapted to generate electromagnetic fieldlines substantially parallel to the charging surface of the primary unitwithin the charging area 740. A primary winding 710 is wound around amagnetic core 750 which may be ferrite or some other suitable material.The charging area 740 includes a series of conductors with instantaneousnet current generally flowing in the same direction. The coilconfiguration of FIG. 6 g is in fact capable of supporting or defining acharging area 740 on both upper and lower faces as shown in the drawing,and depending on the design of the primary unit, one or both of thecharging areas may be made available to secondary devices.

FIG. 6 h shows a variation of the configuration of FIG. 6 g. Instead ofthe primary windings 710 being evenly spaced as in FIG. 6 g, thewindings 710 are not evenly spaced. The spacing and variations thereincan be selected or designed so as to provide improved uniformity ofperformance or field strength levels over the charging area 740.

FIG. 6 i shows an embodiment in which two primary windings 710 as shownin FIG. 6 g are located in a mutually orthogonal configuration so as toenable a direction of the field lines to be dynamically switched orrotated to other orientations about the plane of the charging surface.

FIGS. 6 j and 6 k show additional two-coil configurations for theprimary unit which are not simple geometric shapes with substantiallyparallel conductors.

In FIG. 6 j, line 710 indicates one of a set of current-carryingconductors lying in the plane of the charging surface 600. The shape ofthe main conductor 710 is arbitrary and need not be a regular geometricfigure—indeed, conductor 710 may have straight and curved sections andmay intersect with itself. One or more subsidiary conductors 719 arearranged alongside and generally parallel (at any given local point) tothe main conductor 710 (only two subsidiary conductors 719 are shownhere for clarity). Current flow in subsidiary conductors 719 will be inthe same direction as in the main conductor 710. The subsidiaryconductors 719 may be connected in series or parallel so as to form asingle coil arrangement.

In FIG. 6 k, a set of current-carrying conductors 720 (only some ofwhich are shown for clarity) is arranged in the plane of the chargingsurface 600. A main conductor 710 is provided as in FIG. 6 j, and theconductors 720 are each arranged so as to be locally orthogonal to themain conductor 710. The conductors 720 may be connected in series orparallel so as to form a single coil arrangement. If a first sinusoidalcurrent is fed into the conductor 710, and a second sinusoidal currenthaving a 90° phase shift relative to the first current is fed into thecoil 720, then by varying the relative proportions and signs of the twocurrents a direction of a resultant electromagnetic field vector at mostpoints on the charging area 740 will be seen to rotate through 360°.

FIG. 6 l shows yet another alternative arrangement in which the magneticcore 750 is in the shape of a round disc with a hole in the centre. Thefirst set of current carrying conductors 710 is arranged in a spiralshape on the surface of the round disc. The second set of conductors 720is wound in a toroidal format through the centre of the disc and out tothe perimeter in a radial fashion. These conductors can be driven insuch a way, for example with sinusoidal currents at quadrature, thatwhen a secondary device is placed at any point inside the charging area740 and rotated about an axis perpendicular to the charging area, nonulls are observed by the secondary device.

FIGS. 7 a and 7 b are embodiments of the proposed secondary devices. Awinding 810 is wound around a magnetic core 820. Two of these may becombined in a single secondary device, at right angles for example, suchthat the secondary device is able to effectively couple with the primaryunit at all rotations. These may also be combined with standard coils,as the ones shown in FIG. 2 a 520 to eliminate dead spots.

FIGS. 8 a–8 f show the effect of flux guides 750 positioned on top ofthe charging area. The thickness of the material has been exaggeratedfor the sake of clarity but in reality would be in the order ofmillimeters thick. The flux guides 750 will minimize leakage and containthe flux at the expense of reducing the amount of flux coupled to thesecondary device. In FIG. 8 a, a primary magnetic unit is shown withoutflux guides 750. The field will tend to fringe into the air directlyabove the charging area. With flux guides 750, as shown in FIG. 8 b to 8f, the flux is contained within the plane of the material and leakage isminimised. In FIG. 8 e, when there is no secondary device 800 on top,the flux remains in the flux guide 750. In FIG. 8 f, when a secondarydevice 800 is present with a relatively more permeable material as thecore, part of the flux will flow via the secondary device. Thepermeability of the flux guide 750 can be chosen such that it is higherthan that of typical metals such as steel. When other materials such assteel, which are not part of secondary devices 800, are placed on top,most of the flux will remain in the flux guide 750 instead of travellingthrough the object. The flux guide 750 may not be a continuous layer ofmagnetic material but may have small air gaps in them to encourage moreflux flow into the secondary device 800 when it is present.

FIG. 9 shows an embodiment of a primary unit whereby more than one coilis used. FIG. 9 a shows a coil 710 with a charging area 740 with currentflow parallel to the direction of the arrow 2. FIG. 9 b shows a similarcoil arranged at 90 degrees to the one in FIG. 9 a. When these two coilsare placed on top of each other such that the charging area 740overlaps, the charging area will look like the illustration in FIG. 9 c.Such an embodiment would allow the secondary device to be at anyrotation on top of the primary unit and couple effectively.

FIG. 10 shows an embodiment where the secondary device has an axialdegree of rotation, for example where it is, or is embedded within, abattery cell. In this embodiment the secondary device may be constructedsuch that it couples to the primary flux when in any axial rotation (rA)relative to the primary unit (910), as well as having the same degreesof freedom described above (i.e. translational (X, Y) and optionallyrotational perpendicular to the plane of the primary (rZ)).

FIG. 11 a shows one arrangement where a rechargeable battery cell 930 iswrapped with an optional cylinder of flux-concentrating material 931which is itself wound with copper wire 932. The cylinder may be long orshort relative to the length of the cell.

FIG. 11 b shows another arrangement where the flux-concentratingmaterial 931 covers only part of the surface of the cell 930, and hascopper wire 932 wrapped around it (but not the cell). The material andwire may be conformed to the surface of the cell. Their area may belarge or small relative to the circumference of the cell, and long orshort relative to the length of the cell.

FIG. 11 c shows another arrangement where the flux-concentratingmaterial 931 is embedded within the cell 930 and has copper wire 932wrapped around it. The material may be substantially flat, cylindrical,rod-like, or any other shape, its width may be large or small relativeto the diameter of the cell, and its length may be large or smallrelative to the length of the cell.

In any case shown in FIGS. 10 and 11, any flux-concentrating materialmay also be a functional part of the battery enclosure (for example, anouter zinc electrode) or the battery itself (for example, an innerelectrode).

In any case shown in FIGS. 10 and 11, the power may be stored in asmaller standard cell (e.g. AAA size) fitted within the larger standardcell enclosure (e.g. AA).

FIG. 12 shows an embodiment of a primary unit similar to that shown inFIG. 9. FIG. 12 a shows a coil generating a field in a directionhorizontal to the page, FIG. 12 b shows another coil generating a fieldvertical to the page, and the two coils would be mounted in asubstantially coplanar fashion, possibly with one above the other, oreven intertwined in some fashion. The wire connections to each coil areshown 940 and the charging area is represented by the arrows 941.

FIG. 13 shows a simple embodiment of the Driving Unit (790 of FIG. 5).In this embodiment there is no Control Unit. The PIC processor 960generates two 23.8 kHz square waves 90 degrees out of phase with oneanother. These are amplified by components 961 and driven into two coilcomponents 962, which are the same magnetic units shown in FIG. 12 a andFIG. 12 b. Although the driving unit is providing square waves, the highresonant “Q” of the magnetic units shapes this into a sinusoidalwaveform.

The preferred features of the invention are applicable to all aspects ofthe invention and may be used in any possible combination.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, andare not intended to (and do not) exclude other components, integers,moieties, additives or steps.

1. A primary unit having a power transfer area in or over which asecondary device, separable from the primary unit, can be placed in astationary working disposition to receive power from the primary unitwithout requiring direct electrical conductive contacts between theprimary unit and the secondary device, the primary unit comprising:first and second field generators, each arranged substantially inparallel with said power transfer area and each configured so that ifdriven individually the generator generates an electromagnetic fieldhaving field lines which extend generally in a predetermined directionacross at least part of said power transfer area and which havesignificant resolved components parallel to the power transfer area,said predetermined direction of said first field generator beingdifferent from said predetermined direction of said second fieldgenerator; and a driving unit for applying electrical drive signals tosaid first and second field generators such that the first and secondfield generators cooperate to generate, in or over said at least onepart of the power transfer area, field lines which, when resolved ontothe power transfer area, change in direction over time.
 2. A primaryunit as claimed in claim 1, wherein said driving unit is operable toapply electrical drive signals to said first and second field generatorssuch that said field lines are switched between two or more differentpredetermined directions over time.
 3. A primary unit as claimed inclaim 1, wherein the driving unit is operable to apply electrical drivesignals to said first and second field generators such that the fieldline directions are rotated through an angle over time.
 4. A primaryunit as claimed in claim 1, wherein said driving unit is operable toapply electrical drive signals to said first and second field generatorssuch that the field line directions are rotated through a completecircle over time.
 5. A primary unit as claimed in claim 1, wherein: saidfirst field generator comprises at least one first conductive element;said second field generator comprises at least one second conductiveelement; and said first conductive element(s) extend(s) substantiallyperpendicularly to said second conductive element(s).
 6. A primary unitas claimed in claim 5, wherein: said first and second conductiveelements are arranged in a generating area in or generally parallel tosaid power transfer area; and the first and second conductive elementsare shaped and arranged such that, at each crosspoint where a firstconductive element extends across a second conductive element, the firstand second conductive elements extend in mutually orthogonal directions.7. A primary unit as claimed in claim 6, wherein said first fieldgenerator comprises a plurality of first conductive elements extendinggenerally in parallel with one another and generally in a directionperpendicular to said first direction, and said second field generatorcomprises a plurality of second conductive elements extending generallyin parallel with one another and generally in a direction perpendicularto said second direction.
 8. A primary unit as claimed in claim 7,wherein the driving unit is operable to cause currents to flow in thesame direction simultaneously through all of said first conductiveelements and to cause currents to flow in the same directionsimultaneously through all of the second conductive elements.
 9. primaryunit as claimed in claim 5, further comprising a magnetic core acrosswhich the first and second conductive elements extend.
 10. A primaryunit as claimed in claim 5, wherein each said first conductive elementand/or each said second conductive element is straight.
 11. A primaryunit as claimed in claim 5, wherein each said first conductive elementis not straight and/or each second conductive element is not straight.12. A primary unit as claimed in claim 5, wherein each said firstconductive element is in the form of a loop and each said secondconductive element extends transversely across at least one said firstconductive element at a different position around the loop.
 13. Aprimary unit as claimed in claim 12, wherein at least one said secondconductive element extends transversely across two or more differentfirst conductive elements.
 14. A primary unit as claimed in claim 12,wherein said loop is a generally circular loop and said secondconductive elements extend radially across the circular first conductiveelements at different positions therearound.
 15. A primary unit asclaimed in claim 12, wherein said loop is an irregular loop and eachsaid second conductive element is locally substantially perpendicular toeach first conductive element.
 16. A primary unit as claimed in claim 5,having at least first and second coils, said first conductive elementsbeing provided by different parts of said first coil and said secondconductive elements being provided by different parts of said secondcoil.
 17. A primary unit as claimed in claim 16, wherein said first coilhas a generally flat spiral shape and said second coil is wound in atoroidal form through the centre of the first coil and out to theperimeter thereof in a radial fashion.
 18. A primary unit as claimed inclaim 5, wherein the first conductive elements and the second conductiveelements are conductive tracks formed on one or more planar surfaces.19. A primary unit as claimed in claim 5, wherein said driving unit isoperable to apply electrical drive signals to the first and second fieldgenerators alternately.
 20. A primary unit as claimed in claim 5,wherein said driving unit is operable to apply electrical drive signalsin quadrature to the first and second field generators.
 21. A primaryunit as claimed in claim 1, wherein at any given time said field linesare substantially parallel to one another over said power transfer area.22. A primary unit as claimed in claim 1, wherein said field lines aregenerally parallel to said power transfer surface within said powertransfer area.
 23. A method of transferring power from a primary unit toa secondary device, separable from the primary unit, without requiringdirect electrical conductive contacts between the primary unit and thesecondary device, the method comprising: employing first and secondfield generators to generate an electromagnetic field across a powertransfer area in or over which the secondary device is placed in astationary working disposition to receive power from the primary unit,each of said first and second field generators means being arrangedsubstantially in parallel with said power transfer area and each beingconfigured so that if driven individually the generator generates anelectromagnetic field having field lines which extend generally in apredetermined direction across at least part of said power transfer areaand which have significant resolved components parallel to the powertransfer area, said predetermined direction of said first fieldgenerator being different from said predetermined direction of saidsecond field generator; and applying electrical drive signals to saidfirst and second field generators such that the first and second fieldgenerators cooperate to generate, in or over said at least one part ofthe power transfer area, field lines which, when resolved onto the powertransfer area, change in direction over time.
 24. A system fortransferring power without requiring direct electrical conductivecontacts, the system comprising: a substantially flat power transfersurface; a primary unit having a field generator arranged at or beneathsaid power transfer surface for generating an electromagnetic fielddistributed over a predetermined power transfer area of the surface; andtwo or more objects to be powered, physically and electrically separatefrom one another and each separable from the primary unit and eachcarrying a secondary device adapted to receive power inductively fromthe primary unit when the object is placed on the power transfer surfaceso that the object is lying across the surface; wherein said fieldgenerated by said field generator is such that the secondary device ofeach said object can receive power inductively from the primary unitwhen the object is placed anywhere within said power transfer area; thepower transfer area is of sufficient size that the two or more objectscan be placed at different respective positions within the powertransfer area to enable their respective secondary devices to receivepower simultaneously from the primary unit; and there being nosubstantial discontinuities in the power transfer area, at which one ofsaid objects cannot be placed to receive power, from one side of thearea to the other in two dimensions.
 25. A system as claimed in claim24, wherein there are no substantial discontinuities in the powertransfer area, where one of said objects cannot be placed to receivepower, from one side of the area to the other in two dimensions.
 26. Asystem as claimed in claim 24, wherein field lines of said field aresubstantially parallel to one another at any given moment in time oversaid power transfer area.
 27. A system as claimed in claim 24, whereinfield lines of said field are generally parallel to said power transfersurface at least over a central part of said power transfer area.
 28. Asystem as claimed in claim 24, wherein said primary unit comprises asensing unit which senses one or more of the following: (a) the presenceat the primary unit of a secondary device; (b) the number of secondarydevices present at said primary unit; and (c) the presence at saidprimary unit of magnetic material which is not part of one of thesecondary devices of the system.
 29. A system as claimed in claim 28,wherein the primary unit comprises a control unit which is connectedoperatively to the said sensing unit and which controls a currentsupplied to the field generator in dependence upon the sensingresult(s).
 30. A system as claimed in claim 28, wherein the primary unitcomprises a visual indicator which indicates the presence at the primaryunit of a secondary device.
 31. A system as claimed in claim 28, whereinthe primary unit comprises a visual indicator which indicates the numberof secondary devices present at said primary unit.
 32. system as claimedin claim 24, wherein said field generator comprises a resonant circuit,and the primary unit further comprises a control unit operable tomaintain operation of said resonant circuit at or near resonance as thenumber of secondary devices present at said primary unit changes.
 33. Asystem as claimed in claim 32, wherein said control unit is operable tochange an operating frequency of said resonant circuit so as to maintainsaid operation at or near resonance as the number of secondary devicespresent at the primary unit changes.
 34. A system as claimed in claim32, wherein said control unit is operable to change a capacitance ofsaid resonant circuit so as to maintain said operation at or nearresonance as the number of secondary devices present at said primaryunit changes.
 35. A system as claimed in claim 34, wherein saidsecondary devices are designed such that placing each additional devicelying across the power transfer surface of the primary unit changes aninductance of said resonance circuit to a different one of a pluralityof predetermined inductance values, and said control unit is operable tochange the capacitance of said resonant circuit to one of a plurality ofpredetermined capacitance values suitable for maintaining said operationat or near resonance when the inductance has a corresponding one of saidplurality of predetermined inductance values.
 36. A system as claimed inclaim 34, wherein said control unit has a set of capacitance elementswhich can be selectively switched in or out of said resonant circuit tochange said capacitance thereof.
 37. system as claimed in claim 32,wherein said resonant circuit is configured such that it is detunedautomatically when no such secondary device is present at the primaryunit.
 38. A system as claimed in claim 24, wherein two or more secondarydevices have the same standard size.
 39. A system as claimed in claim24, wherein at least one said secondary device is carried in or by arechargeable battery or cell fitted, or adapted to be fitted, into anobject requiring power.
 40. A system as claimed in claim 39, wherein,when said battery or cell is fitted into said object and said object isplaced lying across said power transfer surface, the battery or cell canbe recharged without removing it from said object.
 41. A system asclaimed in claim 24, wherein at least one said object is a portableelectrical or electronic device.
 42. A system as claimed in claim 24,wherein at least one said object is a mobile communication device.
 43. Asystem as claimed in claim 24, wherein one such object carrying one saidsecondary device has a power requirement different from that of anothersuch object carrying another one of said secondary devices.
 44. Aprimary unit for use in a power transfer system having a power transfersurface shaped and arranged such that a secondary device, separable fromthe primary unit, can be placed on or in proximity to the surface toreceive power from the primary unit without requiring direct electricalconductive contacts between the primary unit and the secondary device,which primary unit comprises: a field generator arranged at or parallelto the power transfer surface for generating an electromagnetic fieldover a power transfer area of the surface, said power transfer areabeing larger than an area of the surface covered by the secondarydevice, and the field being distributed over the power transfer areasuch that a secondary device can receive power from the primary unitwhen placed at any lateral position across the area, at least in onerotational orientation of the secondary device about an axisperpendicular to the surface; and wherein said field generator comprisesone or more conductive elements for generating flux and also comprisesone or more elements made of magnetic material arranged to couple withthe flux generated by the conductive element(s) and distribute the fluxparallel to the power transfer surface, said conductive element(s) andsaid element(s) of magnetic material being shaped and arranged withrespect to said power transfer area such that an instantaneous fieldstrength of said electromagnetic field, measured in said rotationalorientation and at a given distance from the surface, is substantiallythe same at each said lateral position across the area.
 45. A primaryunit as claimed in claim 44, wherein field lines of said electromagneticfield have significant resolved components parallel to said powertransfer surface within said power transfer area.
 46. A primary unit asclaimed in claim 44, wherein there are no substantial discontinuities insaid power transfer area, where the secondary device cannot be placed toreceive power, from one side of the area to the other in two dimensions.47. A primary unit as claimed in claim 44, wherein said field generatorcomprises a plurality of substantially coplanar conductive elementsarranged at or beneath said power transfer surface in a generating area,for generating said electromagnetic field over said power transfer area.48. A primary unit as claimed in claim 47, wherein said substantiallycoplanar conductive elements extend generally in parallel with oneanother across said generating area, and the respective instantaneouscurrents which flow simultaneously through all the conductive elementsin said generating area have the same general direction.
 49. A primaryunit as claimed in claim 47, wherein said conductive elements in saidgenerating area are substantially straight.
 50. A primary unit asclaimed in claim 47, wherein at least one pair of adjacent conductiveelements has a mutual spacing different from that of another pair ofadjacent conductive elements.
 51. A primary unit as claimed in claim 47,further comprising a magnetic core across which the conductive elementsextend.
 52. A primary unit as claimed in claim 47, wherein two or moreof the plurality of conductive elements are different parts of the samecoil.
 53. A primary unit as claimed in claim 52, wherein said coil iswound about a magnetic core.
 54. A primary unit as claimed in claim 47,having further conductive elements outside said generating area whichare connected with said conductive elements within the generating area,the currents flowing through said further conductive elements beinggenerally in the opposite direction to the direction through whichcurrents flow through the conductive elements in said generating area,and the system further comprising magnetic material arranged over thefurther conductive elements for shielding the effects thereof.
 55. Aprimary unit as claimed in claim 47, wherein the plurality of conductiveelements are conductive tracks formed on a planar surface.
 56. A primaryunit as claimed in claim 47, having first and second spiral conductorsarranged side by side, each with parts in the generating area, thecurrent in one of the first and second spiral conductors flowing in ananticlockwise direction at a given moment in time and the current in theother one of the first and second spiral conductors flowing in aclockwise direction at the same moment in time and the plurality ofconductive elements comprising first conductive elements provided bydifferent sections of the first spiral conductor and second conductiveelements provided by different sections of the second spiral conductor.57. A primary unit as claimed in claim 47, having first and second suchgenerating areas arranged side by side, the plurality of conductiveelements comprising first conductive elements in said first generatingarea and second conductive elements in said second generating area, andalso having a spiral conductor with first sections in said firstgenerating area which provide said first conductive elements and withsecond sections in said second generating area which provide said secondconductive elements, whereby an instantaneous current flowing throughsaid spiral conductor flows through each of said first conductiveelements in one direction and through each of said second conductiveelements in the opposite direction.
 58. A primary unit as claimed inclaim 44, wherein said element(s) of magnetic material extend(s) atleast partially around a perimeter of the power transfer and theconductive elements are arranged to generate first and second fluxeswhich are guided by the element(s) of magnetic material to propagate inrespective opposite directions around said perimeter.
 59. A primary unitas claimed in claim 58, having a single element of magnetic materialwhich extends continuously around said perimeter.
 60. A primary unit asclaimed in claim 58, having two said elements of magnetic material thatare opposed to one another and are horseshoe-shaped.
 61. A primary unitas claimed in claim 58, wherein the conductive elements form part offirst and second coils arranged on opposite sides respectively of thepower transfer area.
 62. A primary unit as claimed in claim 61, whereineach said coil is wound around the or one of said element(s) of magneticmaterial.
 63. A system for transferring power without requiring directelectrical conductive contacts, the system comprising: a power transfersurface shaped and arranged such that a secondary device can be placedon or in proximity to the surface to receive power when the system is inuse; a primary unit having a field generator, arranged at or parallel tothe power transfer surface, which generates an electromagnetic fieldover a power transfer area of the surface, said power transfer areabeing larger than an area of the surface covered by the secondarydevice, and the field being distributed over the power transfer areasuch that a secondary device can receive power from the primary unitwhen placed at any lateral position across the area, at least in onerotational orientation of the secondary device about an axisperpendicular to the surface; and a secondary device separable from theprimary unit and including at least one electrical conductor with whichthe electromagnetic field generated by the field generator couples andinduces a current to flow in the conductor when the secondary device isplaced in said at least one rotational orientation at any said lateralposition; wherein said field generator comprises one or more conductiveelements which generate flux and also comprises one or more elementsmade of magnetic material arranged to couple with the flux generated bythe conductive element(s) and distribute the flux parallel to the powertransfer surface, said conductive element(s) and said element(s) ofmagnetic material being shaped and arranged with respect to said powertransfer area such that an instantaneous field strength of saidelectromagnetic field, measured in said rotational orientation and at agiven distance from the surface, is substantially the same at each saidlateral position across the area.
 64. A system as claimed in claim 63,wherein said secondary device comprises a coil which provides said atleast one electrical conductor and which is arranged such that a centralaxis of the coil extends in a predetermined working direction when thesecondary device is placed in said rotational orientation, and a fieldstrength of said electromagnetic field, measured parallel to saidworking direction and at said given distance, is substantially uniformin magnitude at each said lateral position across the power transferarea.
 65. system as claimed in claim 64, wherein said working directionis substantially non-perpendicular to said power transfer surface atleast within said power transfer area.
 66. A system as claimed in claim63, wherein said secondary device comprises a flux-concentrating coreabout which said at least one electrical conductor is wound, said corebeing arranged such that a longitudinal axis thereof extends in aworking direction when the secondary device is placed in said rotationalorientation, and a field strength of said electromagnetic field,measured parallel to said working direction and at said given distance,is substantially uniform in magnitude at each said lateral positionacross the power transfer area.
 67. A system as claimed in claim 66,wherein said working direction is substantially non-perpendicular tosaid power transfer surface at least within said power transfer area.68. A system as claimed in claim 63, wherein said secondary device iscarried in or by an object requiring power and can receive power fromthe primary unit when said object is placed on or in proximity to saidpower transfer surface so that the carried secondary device has its saidrotational orientation.
 69. A system as claimed in claim 63, whereinsaid secondary device is carried in or by a rechargeable battery or cellfitted, or adapted to be fitted, into an object requiring power.
 70. Asystem as claimed in claim 69, wherein, when said battery or cell isfitted into said object and said object is placed on or in proximity tosaid power transfer surface so that said secondary device has its saidrotational orientation, the battery or cell can be recharged withoutremoving it from said object.
 71. A primary unit having a power transferarea in or over which a secondary device, separable from the primaryunit, can be placed in a stationary working disposition to receive powerfrom the primary unit without requiring direct electrical conductivecontacts between the primary unit and the secondary device, the primaryunit comprising: first and second field generating means, each arrangedsubstantially in parallel with said power transfer area and eachconfigured so that if driven individually the field generating meansgenerates an electromagnetic field having field lines which extendgenerally in a predetermined direction across at least part of saidpower transfer area and which have significant resolved componentsparallel to the power transfer area, said predetermined direction ofsaid first field generating means being different from saidpredetermined direction of said second field generating means; anddriving means for applying electrical drive signals to said first andsecond field generating means such that the first and second fieldgenerating means cooperate to generate, in or over said at least onepart of the power transfer area, field lines which, when resolved ontothe power transfer area, change in direction over time.
 72. A system fortransferring power without requiring direct electrical conductivecontacts, the system comprising: a substantially flat power transfersurface; a primary unit having field generating means arranged at orbeneath said power transfer surface for generating an electromagneticfield distributed over a predetermined power transfer area of thesurface; and two or more objects to be powered, physically andelectrically separate from one another and each separable from theprimary unit and each carrying a secondary device adapted to receivepower inductively from the primary unit when the object is placed on thepower transfer surface so that the object is lying across the surface;wherein said field generated by said field generating means is such thatthe secondary device of each said object can receive power inductivelyfrom the primary unit when the object is placed anywhere within saidpower transfer area; the power transfer area is of sufficient size thatthe two or more objects can be placed at different respective positionswithin the power transfer area to enable their respective secondarydevices to receive power simultaneously from the primary unit; and therebeing no substantial discontinuities in the power transfer area, atwhich one of said objects cannot be placed to receive power, from oneside of the area to the other in two dimensions.
 73. A primary unit foruse in a power transfer system having a power transfer surface shapedand arranged such that a secondary device, separable from the primaryunit, can be placed on or in proximity to the surface to receive powerfrom the primary unit without requiring direct electrical conductivecontacts between the primary unit and the secondary device, whichprimary unit comprises: field generating means arranged at or parallelto the power transfer surface for generating an electromagnetic fieldover a power transfer area of the surface, said power transfer areabeing larger than an area of the surface covered by the secondarydevice, and the field being distributed over the power transfer areasuch that a secondary device can receive power from the primary unitwhen placed at any lateral position across the area, at least in onerotational orientation of the secondary device about an axisperpendicular to the surface; and wherein said field generating meanscomprises one or more conductive elements for generating flux and alsocomprises one or more elements made of magnetic material arranged tocouple with the flux generated by the conductive element(s) anddistribute the flux parallel to the power transfer surface, saidconductive element(s) and said element(s) of magnetic material beingshaped and arranged with respect to said power element(s) of magneticmaterial being shaped and arranged with respect to said power transferarea such that an instantaneous field strength of said electromagneticfield, measured in said rotational orientation and at a given distancefrom the surface, is substantially the same at each said lateralposition across the area.
 74. A primary unit as claimed in claim 44,wherein substantially all of the conductive elements within the powertransfer area form part of the same coil.
 75. A primary unit fortransferring power to at least one secondary device without requiringdirect electrical conductive contacts, the or each secondary devicebeing separable from the primary unit, which primary unit comprises: apower transfer surface shaped and arranged such that a secondary devicecan be placed on or in proximity to the surface to receive power whenthe primary unit is in use; and a field generator arranged at orparallel to the power transfer surface for generating an electromagneticfield over a power transfer area of the surface, said power transferarea being larger than an area of the surface covered by the secondarydevice, and the field being distributed over the power transfer areasuch that a secondary device can receive power from the primary unitwhen placed, in at least one rotational orientation of the secondarydevice about an axis perpendicular to the surface, on the surface at anyposition within the power transfer area or in proximity to the surfacealong a normal to the surface that passes through any such positionwithin the power transfer area; wherein said field generator comprisesat least one conductive element for generating flux and also comprisesmagnetic material arranged at or beneath substantially the whole of thepower transfer area for coupling with the flux generated by the at leastone conductive element and distributing the flux parallel to the powertransfer surface, such that a field strength of said electromagneticfield, measured at the surface or at a given distance therefrom and in apredetermined working direction applicable to the position concerned, issubstantially uniform in magnitude at substantially all said positionswithin the power transfer area.